1. Field of the Invention
The present invention relates to an electromagnetic transponder, that is, a transceiver (generally mobile) capable of being interrogated in a contactless and wireless manner by an entity (generally fixed), called a read and/or write terminal. The present invention more specifically relates to transponders having no independent power supply. Such transponders extract the power supply required by the electronic circuits included therein from the high frequency field radiated by an antenna of the read/write terminal, and the data transmitted from the fixed entity to the transponder are transmitted by this high-frequency field in amplitude modulation. The present invention applies to such transponders, be they read-only transponders, that is, transponders adapted to operating with a terminal which only reads the transponder data, or read/write transponders, which contain data that can be modified by the terminal.
2. Discussion of the Related Art
Systems using electromagnetic transponders are based on the use of oscillating circuits including a winding forming an antenna, on the transponder side and on the read/write terminal side. These circuits are intended to be coupled by a close magnetic field when the transponder enters the field of the read/write terminal.
FIG. 1 very schematically shows a conventional example of a data exchange system between a read/write terminal 1 and a transponder 10 of the type to which the present invention applies.
Generally, terminal 1 is essentially formed of a series oscillating circuit formed of an inductance L1 in series with a capacitor C1 and a resistor R1, between an output terminal 2 of an amplifier or antenna coupler (not shown) and a reference terminal 3 (generally the ground). The antenna coupler belongs to a circuit 4 for controlling the oscillating circuit and for exploiting the received data including, among others, a modulator-demodulator and a microprocessor for processing the control signals and the data. In the example shown in FIG. 1, node 5 of connection of capacitor C1 to inductance L1 forms a terminal for sampling a data signal received for the demodulator. Circuit 4 of the terminal generally communicates with different input/output circuits (keyboard, screen, means of exchange with a provider, etc.) and/or processing circuits, not shown. The circuits of the read/write terminal generally draw the power required by their operation from a supply circuit (not shown) connected, for example, to the electric supply system or to batteries.
A transponder 10, intended for cooperating with a terminal 1, essentially includes a parallel oscillating circuit formed of an inductance L2, in parallel with a capacitor C2 between two A.C. input terminals 11, 12 of a rectifying circuit 13 (for example, a fullwave rectifying bridge). The output voltage of bridge 13, sampled across the rectified output terminals 14, 15 thereof, is intended for providing, not only a power supply to electronic data processing circuits 16 (ELEC), but also the very data, modulated in amplitude for a demodulator 17 (DEM).
Since transponder 10 draws its power from the field radiated by terminal 1, it is necessary to provide a circuit 20 for limiting the input voltage of rectifying system 13 that would otherwise risk being damaged by voltages that are too high or carrying these excessively high voltages downstream and thus damaging the electronic circuits. Protection circuit 20 is generally placed as high upstream as possible, that is, upstream of bridge 13. It is, for example, formed of two series-opposition associations of zener diodes 21, 22, 23, 24 with identical thresholds. Being upstream of rectifying bridge 13, a first series-opposition association of zener diodes 21 and 22 is connected between terminal 11 and a ground terminal 25 (for example, confounded with ground terminal 16 at the output of bridge 13). A second series-opposition association of zener diodes 23 and 24 is connected between terminals 12 and 25.
In the example of FIG. 1, the transponder includes, upstream of bridge 13, a voltage regulation circuit 30, the function of which is to provide as regular a power supply as possible to electronic circuits 16. For example, circuit 30 is formed of a resistor 31 in series with a zener diode 32, between terminals 14 and 15. The midpoint 33 of this series connection forms an output terminal providing an approximately D.C. supply voltage to circuit 16. This supply voltage is smoothed by a capacitor 34 in parallel with zener diode 32, the anode of which is connected to terminal 15 and the cathode of which is connected to terminal 33. It should be noted that another capacitor 18 is generally directly connected between terminals 14 and 15 to smooth the voltage transmitted to demodulator 17, as will be seen hereafter.
The transmission of information from transponder 10 to terminal 1 is generally performed by modifying the load formed by this transponder on the terminal's field. A simple way to achieve this is to connect, between terminals 14 and 15, a so-called back-modulation circuit 40. This circuit is, to simplify, formed of a resistor 41 in series with a switch 42 (for example, a MOS transistor), the control terminal of which is connected to electronic circuit 16, and more precisely to the output of a modulator (not shown).
The oscillating circuit of terminal 1 is excited by a high-frequency signal, for example, at 13.56 MHz. The oscillating circuits of terminal 1 and of transponder 10 are generally tuned on the frequency of a transmission carrier corresponding to this high-frequency signal, that is, their respective resonance frequencies are set to a frequency of, for example, 13.56 MHz. This tuning aims at maximizing the power diffusion to the transponder, generally, a card of credit card format or a tag of still smaller format, integrating the different transponder components. The high-frequency remote supply carrier transmitted by terminal 1 is also used as a data transmission carrier. This carrier is generally modulated in amplitude by the terminal according to different coding techniques to transmit the data to the transponder. In response, the back modulation performed by the transponder generally is at a much lower frequency (for example, 847 kHz), which enables the terminal to detect the load variations (be it by amplitude or phase modulation).
FIG. 2 illustrates a conventional example of a data transmission from terminal 1 to a transponder 10. This drawing shows an example of shape of the excitation signal of antenna L1 for the transmission of a code 0101. The modulation currently used is an amplitude modulation with a 106-kbit per second rate (1 bit is transmitted in approximately 9.4 microseconds) much smaller than the frequency of the carrier coming from the transmission oscillator (period of approximately 74 nanoseconds for a 13.56 MHz frequency). The amplitude modulation is generally performed with a modulation rate, defined as being the difference of the peak amplitudes (a, b) between two states (1 and 0) divided by the sum of these amplitudes, much smaller than one due to the need for supply of transponder 10. For example, the modulation rate is on the order of 10%. It should be noted that, whatever the type of modulation used (for example, amplitude, phase, or frequency modulation) and whatever the type of data coding (NRZ, NRZI, BPSK, Manchester, ASK, etc.), the transmission is performed by shifts between two binary levels on the remote supply carrier.
A disadvantage of conventional transponders is that the use of means (20, FIG. 1) for clipping the voltage recovered across the oscillating circuit (L2, C2, FIG. 1) adversely affects the correct data reception in a transmission by amplitude shifts that is not in all or nothing. Indeed, if the transponder is relatively close to the terminal, the voltage is likely to be clipped by circuit 20 in such a way that the transponder demodulator is then incapable of making out a state 0 from a state 1 due to the modulation rate used. Further, this loss of information can occur without having a clipping level lower than the level of state 0 (b, FIG. 2). It is indeed sufficient for the level at state 1 to be clipped to have a risk of interpretation error by the transponder demodulator.
This disadvantage is illustrated by FIG. 3, which shows a simplified example of the shape of voltage V13 across output terminals 14 and 15 (FIG. 1) of rectifying bridge 13 as a function of the inverse of distance d separating the transponder from the terminal. Since the signal shapes of FIG. 3 which will be described hereafter will show an amplitude modulation of the remote supply carrier, it may also be considered that voltage V13 is expressed as a function of time as the transponder progressively comes close to the terminal.
A first curve 26 in dotted lines illustrates the transponder operation in the absence of a regulation circuit 30. In such a configuration, the voltage across capacitor C2 is clipped as soon as threshold V20 of the zener diodes of circuit 20 is reached. Accordingly, it can be considered that from a distance d1 on, the transponder is no longer capable of demodulating the data carried by signal 26 since this signal has turned into a continuous and constant level substantially corresponding to voltage V20 (neglecting the series voltage drop in rectifying bridge 13).
The system operation is improved by the presence of regulation circuit 30. This operation is illustrated by curve 36 in FIG. 3 where the power consumption of circuit 16 is neglected. A first difference with curve 26 is that the presence of resistor 31 in series with zener diode 32 (or with smoothing capacitor 34) causes a voltage drop with respect to the preceding case since the power of the radiated field cannot be modified. As a result, distance d2 at which the clipping appears on the level of voltage V20, downstream of the rectifying bridge, is much closer than distance d1. Accordingly, the operation is maintained for a wider range. However, the presence of this resistor that distributes the power between that for the supply and that for the demodulator attenuates the amplitude available for the demodulator. This attenuation is even greater once zener diode 32 is in avalanche. In FIG. 3, it has been assumed that voltage level V32, corresponding to the threshold of zener diode 32 between terminals 14 and 15 (taking account of resistance 31) is reached for a distance d0. As long as this distance has not been reached, that is, as long as the transponder is further away from this terminal than this threshold, the amplitude attenuation of the modulation performed by resistor 31 is relatively low and can be neglected. Curves 26 and 36 are confounded for distances greater than d0 (left-hand portion of FIG. 3). Between distances d0 and d2, diode 32 is in avalanche and the amplitude of the modulation of signal 36 is attenuated. From distance d2 on, the diodes of circuit 20 are in avalanche and the modulation can no longer be detected.
The use of a regulation circuit 30 such as described in FIG. 1 clearly already is an improvement as compared to the simple use of the limiting circuit upstream of the rectifying bridge. However, it is necessary to perform a compromise between the value given to resistance 31 and the so-called “blinding” area, that is, the distance range (distances smaller than d2) in which the transponder can no longer detect data. The greater the value of resistance 31, the closer the shape of the curve will be to shape 26 with no regulator. The smaller the resistance, the more the blinding area is reduced, but the smaller the amplitude of the data modulation between distances d0 and d2.
Another known solution to solve the problem of a voltage varying according to distance consists of limiting the transmission power of the terminal. A disadvantage of such a solution however is that this then limits the transponder system range. Further, the magnetic fields that the transponders are supposed to withstand are most often imposed by standards and the application of the standards currently in force results in that the magnetic field received by the transponder, when its clipping means starts operating, is much smaller than the maximum magnetic field that the transponder must be able to withstand according to standards. Accordingly, the transponder is often supplied by a signal clipped by circuit 20 and the data are then lost.
The above problems are more critical still for low power consumption transponders. Indeed, in this case, the circuits internal to the transponder provided to have a low power consumption are not able to withstand high voltages, so that the clipping means must be sized relatively low.